The two-leg playoff, decided by aggregate goals, will conclude Tuesday at BMO Stadium where LAFC has lost just 12 times in six seasons against MLS opponents.

“It’s 1-1 and we’re going back to L.A. and we have an away goal,” LAFC coach Steve Cherundolo said. “It’s a game we want to win and if we win, we’re through to the final.”

Goalkeeper John McCarthy was LAFC’s hero for the first 82 minutes, making three stout saves to keep the game scoreless. But that was wasted when Jack Mc-Glynn’s shot from just outside the box struck Acosta in the arm, leading Mexican referee César Ramos to award a penalty after a lengthy video review.

McCarthy, who stopped two penalties in the tie-breaking shootout in LAFC’s MLS Cup win over the Union last November, guessed wrong on Daniel Gazdag’s try, allowing the Union to take a 1-0 lead.

On the other end Philadelphia keeper Andre Blake was tested just once during regulation time, that coming in the 66th minute when Denis Bouanga got a boot on a long José Cifuentes cross from the touchline and directed it toward the goal, only to see Blake lay out to make the save, then grab Bouanga’s back-heel attempt on the rebound.

But LAFC came alive in stoppage time.

A poor clearance attempt by Philadelphia struck the Union’s Leon Flach and caromed to Acosta, who laid the ball off and sprinted into the box. Timothy Tillman found him there with a right-footed cross that Acosta, who was going to the turf, reached back and deflected over a perfectly positioned Blake on a high bounce to tie the score and keep the series even.

“We are ecstatic with the result, obviously not the performance,” Cherundolo said. “But one thing about this team, they find ways and they never give up and they have the quality to put these goals in in these very small chances.”